The invention broadly relates to a catheter and a method for its use in opening a stenosis in a coronary artery or any other vascular vessel. In particular, the invention relates to an inflatable balloon catheter wherein the balloon also serves as a piezoelectric generator of ultrasonic energy for breaking up the stenosis.
Percutaneous transluminal angioplasty (PTA) is presently the primary therapy for certain forms of atherosclerotic artery disease. The major application of the procedure is in the coronary arteries, but the procedure can also be used in peripheral arteries or any vascular vessel. Atherosclerosis results in the restriction and blockage of blood flow in arteries by an accumulation in the blood vessel of a variety of biological materials. Such restriction or blockage results in oxygen deprivation of the tissue supported by the blood supply. This deprivation and its effect, angina, is referred to as "ischemia". If the blood supply through the coronary artery is almost completely or completely blocked for more than two or three minutes, permanent damage to the myocardia or infarction and death may result. The biological matter causing arterial blockage (stenosis) may be plaque, or thrombotic, calcific or fibrous matter or any combination thereof.
Several methods are known in the art to dilate an existing path through a stenosis and restore blood flow. Balloon angioplasty requires the insertion into the blood vessel and through the stenosis of a deflated balloon, which is hydraulically inflated to stretch and compact the stenosis material against the wall of the artery. This procedure is somewhat effective; however, the incidence of re-stenosis is high, and in many cases bypass surgery must be undertaken. A confirmed attendant risk associated with this procedure is subsequent downstream embolism or clogging, which can be essentially as serious as the most serious stenosis which the procedure is designed to remedy.
In order that the smallest possible stenosed aperture may be crossed, conventional balloon catheter designs call for a balloon having minimal profile diameter in the deflated state. As a result, the balloon wall thickness must be minimized, reducing the burst pressure of the balloon. At the same time, however, there is a growing demand for higher balloon burst pressures to overcome the high resistance of some biological materials in stenoses that are somewhat or completely calcified.
The need for a minimal profile diameter in the deflated state also requires a minimization of the catheter shaft tip to as small a diameter as can accommodate a given guide wire. However, this impairs the pushability of the catheter across the stenosis, since a thinner shaft is also a weaker shaft.
Most balloons are made of either polyethylene, polyethylene terephthalate (PET), or a polyolefin copolymer. A conventional balloon catheter comprises a balloon-over-a-wire design, however this invention anticipates on-the-wire piezoelectric catheters where the guide wire occupies a central lumen of the catheter.
A recent development in treatment of stenosis is the use of ultrasonic energy to break up the biological material comprising stenosis in peripheral blood vessels. The mechanisms of ultrasound treatment are primarily direct mechanical effects and cavitation. Generally, the ultrasonic energy is generated in vitro and delivered, for example, via a titanium wire of 0.5 millimeter diameter to the 2 millimeter spherical tip of the catheter at the site of stenosis. Frequencies in the range of 10-20 kHz are typically used with a power output of up to 20-25 W/cm.sup.2. Such a device is described in R. J. Siegel et al., "Percutaneous Ultrasonic Angioplasty: Initial Clinical Experience", Lancet, pp 772-774, Sept. 30, 1989.
The spherical tip must be large enough to create a pathway through the stenosis through which a subsequent angioplasty balloon catheter may pass. Unfortunately, though the spherical tip breaks open a lumen of approximately 2 millimeter diameter through the stenosis, it does not remove the annulus of biological material which surrounds the 2 mm aperture. Instead the annulus must be crushed against the vessel wall by the subsequent angioplasty balloon in order to achieve a larger aperture.
Another problem with the conventional catheters used to mechanically deliver ultrasonic energy is that the titanium wire which is generally used for this purpose is relatively stiff and therefore cannot be effectively steered into a coronary artery. A thinner, more flexible wire, such as stainless steel, is not able to transmit effectively the amount of ultrasonic energy required by the procedure. Titanium is thus the material of choice in the mechanical transmission of the ultrasonic energy. Consequently, this method of delivering ultrasonic energy for ablation of the stenosis cannot be applied to the coronary arteries.
Diagnostic catheters are known in the art which have in vivo piezoelectric transducers at their tips. These piezoelectric transducers are used for ultrasonic imaging of the vessel in which the catheter is inserted. Often, such transducers are used in combination with an angioplasty balloon catheter. In such a combination, the transducer provides imaging of the stenosis for which the balloon is used to dilate. The transducer may also be used to measure the flow in an autoperfusion catheter, in which a conduit is incorporated in the catheter to allow blood to flow by the inflated balloon. Such a catheter is disclosed in U.S. Pat. No. 5,046,503 to Schneidermann. The piezoelectric transducer is a piezoelectric crystal placed adjacent the autoperfusion conduit inside the balloon.
A different kind of piezoelectric element can be found in the intravascular, ultrasonic imaging catheter of U.S. Pat. No. 5,109,861 to Walinsky et al. Thin layers of flexible plastic material, such as polyvinylidene diflouride (PVDF), are utilized which can be spot polarized in active regions to exhibit piezoelectric characteristics. This kind of imaging catheter may be used in conjunction with balloon catheters to assist in locating the portion of the vessel wall to be treated. However, the low amplitude, high frequency piezoelectric transduction of ultrasonic energy cannot, and is not intended to, ablate the stenosis under observation.
U.S. Pat. No. 5,135,001 discloses a piezoelectric element in the form of a PVDF layer sandwiched between an inner cylindrical electrode and a plurality of outer electrode strips running axially along the length of the catheter, for use in imaging the inside surface of the blood vessel. In one embodiment, the piezoelectric element is contained within an inflatable balloon. After the catheter is positioned at the desired location, the balloon may be inflated with liquid until it contacts the vessel walls. This assures more efficient transmission and echo reception of ultrasound energy than would be possible if there were gaps between the catheter sheath and the vessel walls. In alternative embodiments, the outer electrode strips are located on the outer surface of the balloon, while the balloon still contains the inner cylindrical electrode and the piezoelectric layer. The electrode strips may also be attached to the inside of the inflatable balloon.
While it has been demonstrated that ultrasonic energy can be used to image the inside surfaces of blood vessels, and ultrasonic energy has been delivered over titanium wires to stenoses in peripheral vessels to open up a nominal aperture, no effective means of delivering ultrasonic energy to substantially ablate stenosis material in a coronary artery as provided by the present invention is presently available. There is a need for a catheter which can deliver ultrasonic energy to the location of stenosis, to substantially open the blockage without the need to use balloon pressure to crush the stenosis open. There is a further need to avoid the high risk of re-stenosis which accompanies balloon angioplasty, and to ablate the stenosis material in particle sizes sufficiently small to reduce the risk of downstream embolism.